Monoesters such as monoglycerides are widely used food additives, for example as emulsifiers and dough conditioners. In general, such materials comprise esters of fatty acids. The term "monoglyceride" specifically refers to a derivative of glycerol, i.e., a glyceride, in which only one of the three available hydroxy groups of the glycerol moiety is esterified, hence the prefix "mono-". By "esterified" in this context, it is meant that the glycerol moiety forms the alcohol residue of an ester (typically with a fatty acid residue).
In general, crude monoglyceride mixtures are made from reacting naturally occurring triglycerides, often obtained from oil seed processing, with glycerol. Such reactions generate a mixture of monoglycerides, diglycerides and triglycerides. Limitation on monoglyceride production, via this approach, is generally controlled by: (1) solubility of the glycerol in the reaction mixture; (2) the overall equilibria statistics; and, (3) time. Typical commercially available crude monoglyceride mixtures made using this approach include ratios of monoglyceride:diglyceride:triglyceride (by weight) of about 45:45:10; or about 60:35:5, depending on processing conditions used.
In many instances, it is preferred to utilize more purified monoglycerides. That is, crude monoglyceride compositions or mixtures are purified for at least partial isolation of the monoglycerides from the diglycerides and triglycerides. In general, monoglyceride distillation has been the most widely utilized technique for such purifications. Typically the crude monoglyceride mixture is distilled under vacuum, in a short path distillation process. The distillate generally comprises greater than 90% (by weight) monoglycerides. The remainder generally comprises diglyceride. During the process, the monoglycerides are generally heated to at least 200.degree. C.
In other processes, supercritical extractions have been used for isolation of monoglycerides. These generally concern extraction under pressures greater than atmospheric (typically 30-80 atmospheres) and temperatures in excess of 100.degree. C. (typically 110.degree. C. or so). They generally concern extractions with low molecular weight hydrocarbons, such as propane. Such approaches are generally prohibitively expensive, for application on a large scale.
Closely related materials to monoglycerides include propylene glycol monoesters (PGME's). Such materials are generally made from esterifying propylene glycol with naturally occurring oils, i.e., fatty acid mixtures, resulting in a mixture of monoesters and diesters. Generally the monoesters are isolated by distillation. Such materials are also widely utilized as emulsifiers and as dough conditioners in the food industry.
In certain food applications, diglycerides are used. For example, diglycerides may be used as components of shortening and as fat replacers. As a result, in some instances, it is desirable to isolate diglycerides in a purified form, from a crude mixture. Techniques to accomplish this are described herein. Herein the term "diglyceride" refers to a derivative of glycerol in which two of the three available hydroxy groups of the glycerol are esterified; and, the term "triglyceride" refers to a derivative of glycerol in which all three available hydroxy groups are esterified.